This invention pertains to incandescent lamps and more particularly to an incandescent lamp that includes an envelope having an ellipsoidal portion with a coating that reflects a selected portion or spectrum of emitted light toward a filament housed within the envelope. The invention finds particular applicability in the automotive industry for use in a headlamp, although the invention may find applicability in still other environments and applications.
A typical headlamp assembly includes a light source or lamp located at a focus of a reflector so that light emitted from the light source is directed through a lens of the headlamp assembly to achieve a predetermined beam pattern. An ideal lamp for use in the headlamp assembly is a halogen infrared light source in which a portion of the emitted radiation is reflected by a coating on the light source envelope toward the filament to achieve a goal of increased efficiency of the lamp. The reflection is achieved through use of an ellipsoidally shaped portion of the envelope having an external interference filter or coating preferably comprised of alternating layers of two or more materials having different refractive indexes. The filament is substantially aligned with the longitudinal axis of the envelope and the coating is intended to selectively transmit portions of the visible radiation emitted by the filament while reflecting other portions of the radiation.
For example, a multi-layer coating of tantala and silica material provided on a quartz envelope with the filament substantially coincident with the major axis of the ellipsoidal portion reflects infrared radiation toward the filament. The infrared energy returned to the filament further heats the filament to improve the overall efficacy of the lamp. To maximize the output of the light source, it is important to align the filament with the major axis of the ellipsoidal portion of the envelope. Ideally, a filament located directly on the center line of the ellipse would maximize the temperature of the filament in response to the reflected infrared radiation.
A second concern with headlamp assemblies is to eliminate glare from the projected beam pattern. One cause of glare is the off-center location of the filament relative to the major axis of the ellipsoidal portion of the envelope. A virtual image is formed on the opposite side of the axis as a result of radiation reflected by the envelope coating. This virtual image is at a location that results in glare and can adversely affect the desired beam pattern.
If the filament is substantially offset relative to the major axis, glare resulting from a virtual image becomes less of a problem. However, when the competing concern is to locate the filament substantially on axis, and the virtual image forms at a region offset but close to the major axis, glare becomes a particular problem. In reality, the filament is rarely located directly on a major axis and these competing concerns must be addressed in a manner that satisfactorily resolves both goals.